1. Field of the Invention
The invention relates generally to electrical apparatus and more particularly to circuit interruptors for use with AC secondary network systems.
2. Description of the Prior Art
In supplying the nation's energy needs, two primary objectives of the electric utility industry are safety and reliability. Since the late 1920's the AC secondary network system has been used in downtown business districts and commercial areas to provide a high degree of service continuity. In the AC secondary network system, the secondary mains surrounding the area being served, such as a city block, are connected together to form a secondary network grid or mesh at low voltage from which the customer loads are supplied. The secondary network is supplied from a plurality of high voltage transmission lines or feeders through network transformers. The transformers reduce the high voltage necessary for transmission of electric energy to lower voltage suitable for distribution to customers. In the AC secondary network system the failure of any one feeder will not cause interruption to service because the load will be supplied over the remaining feeders. When a fault occurs in a high voltage feeder or in one of its associated network transformers, the input end of the feeder is disconnected from the system by opening of the feeder circuit breaker. However, it is also necessary that all network transformers on the faulted feeder be disconnected from the network by some form of protective device to prevent power from the network from being fed back through the network transformer to the fault. The automatic network protector was developed for this purpose. The network protector consists of a specially designed air circuit breaker with a closing and opening mechanism controlled by a network master relay and a network phasing relay. When the network protector is closed, the master relay functions to trip it when a reversal of power flow occurs. The master relay and the phasing relay act together to close the protector when, and only when, the correct voltage conditions exist across it.
Network protectors are often located outdoors or underground in vaults and must be protected by a sealed enclosure. When approached for service, testing, or maintenance, network protectors must be electrically and physically disconnected from the system on both the transformer and the load side. Historically, this consideration dictated the use of a rollout type circuit breaker which could be disconnected and rolled out of its enclosure for inspection and maintenance. Performing maintenance on prior art network protectors presented potential hazards to maintenance personnel since a piece of hardware, a tool, a fuse, or a disconnect link might be accidentally dropped into the protector and come in contact with components at high potential, causing a flash or arc-over and producing injury to maintenance personnel. Thus, it was desirable to remove the protector to a maintenance area in order to perform the required operation. The unitary construction of prior art network protectors made it difficult to perform maintenance upon a section of the network protector without taking the entire unit out of service. In order to replace a worn set of contacts, for example, it was necessary to remove the entire draw-out unit to the shop. This required either that a spare network protector be installed or that the network be served by one less feeder. Thus, it would be desirable to provide a circuit interruptor of modular construction which would allow removal of any module from the network protector without requiring removal of other modules. It would also be desirable to provide interlocks such that no module could be removed until the draw-out unit was completely disengaged from sources of high potential.